1. Field of the Invention
This invention relates to telephony systems and more particularly to automatic systems for detecting the off-hook condition of a telephone unit.
2. Description of the Prior Art
U.S. Pat. No. 3,205,312 of Brightman et al entitled "Off-Hook Detector" describes a method of charging a capacitor periodically which discharges through a transformer at a variable rate depending upon whether the telephone is on or off the hook. The potential on the capacitor is monitored periodically to compare its potential with a predetermined value.
U.S. Pat. No. 3,720,793 of Hofmann entitled "Supervisory Circuit Arrangement for Telephone Subscriber Lines" includes an off-hook sensor which uses mechanical transfer relays, which involve the usual type of high energy use, mass, copper requirements, size and reliability problems to sample the surging current during the first part of each positive half-wave of a ringing signal.
U.S. Pat. No. 3,835,258 of Le Cardonnel entitled "Ring Trip Circuit" employs a filter F, oscillator O, transformer TR, a rectifier RE and a switching device CO with a transistor switch which operates a relay O.
U.S. Pat. No. 3,735,263 of Boatwright entitled "Digital Analysis of Electric Wave Signals" teaches producing a fixed number of pulses per cycle of the incoming waveform for measuring the duty cycle of only a repetitive signal. A sum of the total duration of the period of a signal is accumulated into a first register. A second series of pulses is accumulated into a second register in an amount equal to the duration of the active portion of the signal. Then a reversible counting step for counting down the second register for the active portion while measuring time with pulses from the first register. At least two full cycles of operation are required for the result to be obtained which includes a substantial built-in time delay in achieving the desired result.